Researchers reveal a little more of the TRAPPIST-1 story

A size comparison of the planets of the TRAPPIST-1 system, lined up in order of increasing distance from their host star. The planetary surfaces are portrayed with an artist’s impression of their potential surface features, including water, ice, and atmospheres. Credit: NASA/R. Hurt/T. Pyle

Barely a year after NASA grabbed our attention by announcing it had discovered no less than seven potentially habitable Earth-sized worlds around a nearby star, we now have a better idea of exactly what they found.

An international team has published the results of four studies that examined the properties of the TRAPPIST-1 planetary system, which many consider our most optimal hope for evidence of biological life beyond the solar system.

The researchers refined the properties of the star at the centre of the system, improved the measurements of the planets’ radii, made better estimates of their masses and did some reconnaissance work on their atmospheres.

Exoplanets considered temperate

“When we combine our new masses with our improved radii measurements and our improved knowledge of the star, we obtain precise densities for each of the seven worlds,and reach information on their internal composition,” says University of Birmingham astronomer Dr Amaury Triaud.

Currently TRAPPIST-1e, the fourth from the star, is considered the most akin to Earth, although much remains to be known, notably the conditions at the surface and whether it holds an atmosphere.

All seven are considered temperate, meaning that under certain geological and atmospheric conditions all could possess conditions allowing water to remain liquid. Work is now proceeding to pinpoint which is most likely to be habitable.

No evidence for hydrogen-dominated atmospheres

All are made mostly of rock, with up to 5 percent of their mass in water; by comparison, Earth’s oceans account for only 0.02 per cent of it mass. Five appear devoid of an atmosphere made of hydrogen and helium, like for Neptune or Uranus.

As part of its work, the team used the Hubble Space Telescope, attempting to catch minute signals while starlight interacted with the planets’ atmospheres.

Their careful measurements found no evidence for hydrogen-dominated atmospheres on planets TRAPPIST-1d, e and f (b and c were done last year) although the hydrogen-dominated atmosphere cannot be ruled out for g.

So far, the collected data is still consistent with, but cannot confirm whether the planets have atmospheres similar to Venus, or Earth. This identification will be carried out by new observations.

“Thanks to our efforts, the TRAPPIST-1 planets are becoming the best studied worlds outside the Solar system,” Triaud says.

The next step in the quest for knowledge will involve NASA’s James Webb Space Telescope. It will be able to delve into the question of whether these planets have atmospheres, if so what those atmospheres are like, and whether they allow adequate surface conditions to permit liquid water.

What is TRAPPIST-1?

TRAPPIST-1 is named for the Transiting Planets and Planetesimals Small Telescope (TRAPPIST) in Chile, which discovered the first two of the seven planets in 2016. NASA’s Spitzer Space Telescope, in collaboration with ground-based telescopes, confirmed these planets and uncovered the other five.

Since then, NASA’s Kepler space telescope also has observed the TRAPPIST-1 system and Spitzer has collected additional data.

The TRAPPIST-1 planets huddle so close to one another that a person standing on the surface of one of these worlds would have a spectacular view of the neighbouring planets in the sky, which would sometimes appear larger than the Moon looks to an observer on Earth.

They may also be tidally locked, meaning the same side of the planet is always facing the star, and each side is in perpetual day or night. Although the planets are all closer to their star than Mercury is to the Sun, TRAPPIST-1 is such a cool star that its planets are temperate.

About the Author

Nick Carne
Nick Carne is the Editorial Manager for the Royal Institution of Australia.

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